1. Field of the Invention
The sealing cover unit fabricated in accordance with the method of the present invention is of particular utility in hermetically sealing a container for a semiconductor device such as an integrated circuit which must be protected from exposure to ambient atmosphere.
2. Description of the Prior Art
In the manufacture of semiconductor devices, there has been a requirement for hermetically sealing the container in which the active semiconductor device is housed, usually a cavity in a metallic or ceramic body. For this purpose, a metallic cover and a solder preform ring have been used. Initially, the metallic cover and the solder preform ring were assembled separately and disposed on a sealing ring of a container and the assembly was heated to solder the cover to the container and hermetically seal the container.
In accordance with current general practice, the solder preform ring is preattached to the sealing cover and the resulting unit disposed over the container of the semiconductor device and heated to fuse the solder preform to the cover and to the container as described and claimed in applicant's U.S. Pat. Nos. 3,823,468 and 3,874,549. Such preattached sealing cover-solder preform units have acquired substantial commercial success since they have several important advantages over the use of separate sealing covers and solder preforms, namely, increasing substantially the hermetic sealing yields, lowering assembly costs, and alleviating the logistics of keeping balanced inventories of the sealing covers and the solder preforms for various sizes of semiconductor packages.
In the current practice of fabricating the preattached cover element and solder preform, the latter usually has a thickness of 0.001 inch to 0.005 inch and is stamped from a continuous solder ribbon of the desired gold-tin eutectic alloy. As a result, approximately 15% of the ribbon resides in the solder preform and 85% becomes scrap in the form of a skeleton and the center punched-out sections. Ordinarily, the gold or other precious or semiprecious metals in the scrap are recycled by melting, adjusting the alloy content to the required ratio, recasting, rolling and reusing for manufacturing additional solder preforms. Often, the alloy becomes unusable for recycling because of the accumulation of foreign materials. The solder must then be refined into its various pure metal components. A so-called virgin melt must then be made from the refined metals.
Such recycling and refining operations entail appreciable labor, material, and energy costs. Some of the precious metal is necessarily lost in the refining and remelting steps of manufacture and this adds additional cost. A substantial economic disadvantage of obtaining such a low yield of the solder ribbon in the finished solder preform is the requirement that an excessive amount of precious metal--usually gold--must be tied up in inventory and work in process. The interest costs on the excessive amount of precious metal that must be purchased are substantial.
It is an object of the invention to provide an improved method and apparatus for fabricating a novel sealing cover-solder preform unit.